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Pharmacology RCOA > Opioids > Flashcards

Opioids Flashcards

1
Q

How does nociceptive pain occur?

A
  • Stimulation of nociceptors afferent fibres (Aδ, C) by initiating mediators (H+, K+, ACh, 5HT, Histamine) and sensitising mediators (Prostaglandin, Leukotrienes, Neurokinin)
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2
Q

What are Aδ fibres?

A

Small myelinated
Conduct sharp pain rapidly

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3
Q

What are C fibres?

A

Unmyelinated
Conduct dull pain slowly
Enter dorsal horn & synapse
Substantia gelatinosa integrate inputs

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4
Q

How do opioids inhibit pain?

A
  • Bind to inhibitory G-proteins
  • Voltage-sensitive Ca2+ Channels close
  • Hyperpolarization by K+ efflux
  • Adenylase cyclase inhibition
  • Reduced cAMP
  • Inhibit transmission between nerves
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5
Q

Where are µ receptors and what do they do?

A
  • Throughout CNS, primary afferents in dorsal horn
  • Resp depression (reduce chemoreceptor sensitivity to CO2)
  • Constipation (reduce secretions & peristalsis)
  • CV depression
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6
Q

What is Morphine?

A

Naturally occurring Phenanthrene derivative
µ- receptor agonist

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7
Q

What are the effects of Morphine?

A
  • Effective for visceral pain
  • Histamine release & bronchospasm
  • 5HT & dopamine stimulating = vomiting
  • Pruritis marked following epidural/intrathecal
  • Meiosis (Edinger-Westphal nucleus)
  • Inhibits prolactin, ACTH, GnT
  • Inc ureteric tone & retention
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8
Q

How is morphine absorbed?

A
  • Ionised in gastric acidity
  • Absorption delayed until reaches small bowel (alkaline) = unionised
  • Undergoes 1st-pass metabolism to morphine-3-glucuronide
  • Conc in brain falls slowly due to low lipid solubility
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9
Q

What is diamorphine?

A

Diacetylated morphine derivative
No affinity for opioid receptors
Prodrug

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10
Q

How is diamorphine absorbed?

A
  • High lipid solubility, well absorbed from gut
  • Low PO bioavailability due to high 1st-pass
  • Metabolised in liver, plasma, CNS by ester hydrolysis
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11
Q

How is Codeine absorbed?

A
  • Due to methyl group reduced 1st pass metabolism
  • Eliminated by 1 of 3 liver pathways
    1) 6-hydroxyglucoronidation
    2) N-demethylation
    3) O-demethylation (depends on CYP2D6)
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12
Q

What is Fentanyl?

A

Synthetic phenylpiperidine derivative
Rapid onset
µ-receptor agonist

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13
Q

Why does an epidural Fentanyl bolus not cause resp depression?

A

High lipid solubility
Rapidly diffuses out of CSF into spinal cord

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14
Q

How is Fentanyl absorbed?

A
  • High lipid solubility = rapid onset
  • Short duration of action due to distribution
  • Can become trapped in the stomach where 99% ionised
  • Passes into alkaline s.bowel for absorption
  • Rapid hepatic 1st-pass (N-demethylated)
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15
Q

What are Pethidine, Remi & Alfentanil?

A

Synthetic phenylpiperidine derivative
µ-receptor agonist

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16
Q

How is Alfentanil absorbed?

A
  • 89% in unionised form, able to cross lipid membranes
  • Faster onset of action
  • Metabolised by N-demethylation
17
Q

What drugs can increase the 1/2 life of Alfentanil?

A
  • Midazolam: Also metabolised by CYP3A3/4)
  • Erythromycin: Inhibits CYP450
18
Q

How is Remi absorbed?

A
  • Duration of action determined by metabolism NOT distribution
  • Rapidly broken down by non-specific plasma & tissue esterases
19
Q

What drug group should Pethidine NOT be given with?

A

MAOIs
Central serotoninergic hyperactivity
Coma, seizure, hyperpyrexia, labile circulation

20
Q

How is Pethidine absorbed?

A
  • More lipid soluble than morphine
  • Metabolised by liver ester hydrolysis
  • Metabolite derivative = renal failure, hallucinations, seizures
21
Q

What is the MOA of Methadone?

A
  • Opioid antagonist
  • NMDA receptor antagonist - Tx certain neuropathic pain resistant to opioids
22
Q

What is Tramadol?

A

Cyclohexanol derivative

23
Q

What is the MOA of Tramadol?

A
  • Agonist of all opioid receptors ( particularly µ)
  • Inhibits reuptake of NorA & 5HT
  • Stimulates presynaptic 5HT release
24
Q

What is a contraindication to Tramadol

A

Epilepsy

25
Q

How is Tramadol absorbed?

A

High oral bioavailability
- Metabolised by hepatic CPY2D6, CYP3A4, CYP2B6

26
Q

What receptors dose Naloxone work at?

A

µ, δ, κ
Highest affinity to µ

27
Q
A

Pharmacology RCOA (12 decks)

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